A monolithic twopole crystal filter comprises two acoustically coupled resonators on one crystal wafer. The resonators are formed by two pairs of opposite electrodes, separated by a gap on at least one side of the wafer. Fine tuning the filter usually entails adjusting the center frequency and, preferably, also the bandwidth of the filter. The center frequency is determined by the two resonator frequencies, which can be adjusted by changing the mass of the electrodes. The bandwidth is determined by the inter-resonator coupling and can be adjusted by changing the mass distribution over the area covered by the electrodes and the inter-electrode gap. For example, the bandwidth can be increased by depositing additional mass in the gap or its vicinity, such as the electrode edges adjacent to the gap. If the filter has only one gap, the bandwidth can also be increased by increasing the mass opposite the gap on the wafer side opposing the gap. Alternately, the bandwidth can be decreased by subtracting mass in the described areas or by adding mass to the electrode edges furthest removed from the gap. Changing the mass can be achieved by various means, such as mass addition by vacuum deposition or chemical action, and mass removal by laser.
In the following, several conventional Methods for fine tuning of monolithic crystal filters are described. They are all based on adjusting some of the characteristic series resonance frequencies of the twopole, which are defined as follows: an upper and lower short circuit frequency for each side of the twopole, each measured with the opposite side short circuited; an upper and lower open circuit frequency for each side of the twopole, each measured with the opposite side open circuited; a resonator frequency for each side of the twopole, each measured with the opposite side's static capacitance neutralized. These definitions will become more apparent by reference to the twopole equivalent circuit, which is explained farther below. If the twopole is symmetric, each characteristic frequency of one side equals the corresponding one on the other side.
Method 1 is based on U.S. Pat. No. 4,343,827, which pertains to fine tuning by using a transmission method for visually displaying and adjusting the short circuit amplitude response versus frequency. First, the resonator frequencies are equalized by balancing the amplitude peaks at the two short circuit frequencies by means of adding mass to the electrodes. Then, both resonator frequencies, which are located midway between the two short circuit frequencies, are lowered to target by further mass addition. The method does not allow adjustment of bandwidth.
Method 2 is based on Method 1 but includes fine tuning of the bandwidth, which is defined as the difference between the two short circuit frequencies and is adjusted by changing mass at the electrode gap or the electrode edges.
Methods 1 and 2 require both frequency and amplitude adjustment and have limited accuracy because they depend on judging frequencies from a visually displayed curve.
Method 3 is used in commercial equipment such as the "Twopole Plating System" by Transat Corp. It is based on oscillator measurement of the two resonator frequencies and the bandwidth, the latter being measured by a network that uses two oscillators for simultaneous oscillation at the upper and lower short circuit frequencies and subsequent mixing and filtering of these frequencies to monitor the bandwidth.
Method 4 is based on U.S. Pat. No. 4,093,914, which describes a "Method of Measuring Parameters of a Crystal Filter" by means of measuring the two short circuit and two open circuit frequencies for one side of a twopole and then calculating the two resonator frequencies and the bandwidth. The method is attractive because of its simple measurement but is apt to have increasing accuracy limitations toward higher frequencies because of (a) the complex relationship between the measured and targeted frequencies, (b) the effect of the stray reactance of the measurement network on accuracy, (c) the heightened requirement for accuracy for the equivalent circuit, caused by the measurement from only one filter side.
Both Methods 3 and 4 depend on adjusting four frequencies during the tuning process.
In Methods 1-4 the main means of changing frequency is by way of metal deposition in vacuum, also called "plating". The deposition may be on one side or both sides of the filter wafer, and the selective mass distribution may be accomplished by one or more evaporation sources in conjunction with stationary or movable masking.
In any fine tuning approach the method of measurement is of critical importance, especially with regard to accuracy toward higher frequencies. The main applicable methods are based on oscillator, transmission, or reflection measurements. At present the operating frequency of most mass produced monolithic filters lies below 60 MHz, where oscillator measurement is adequate. As frequency increases further, the transmission or reflection methods are preferable, but they can encounter increasing accuracy limitations as the filter reactances approach the magnitude of the stray reactances of the measurement network. There is a strong need for higher frequency filters (up to at least 200 MHz), yet there appears to be no suitable commercial method available for efficient and accurate fine tuning of both center frequency and bandwidth. The present invention stems from the search for such a method and is based on two main objectives:
1. Find a wide band fine tuning method based on monitoring and adjusting only three frequencies instead of the four frequencies of prior art methods.
2. Find a measurement method which has minimum sensitivity to the effect of external stray reactances on the twopole measurement.